1. Field of the Invention
The present invention relates to a wireless communication apparatus and a method thereof, and more particularly, to a wireless communication apparatus capable of adjusting a beacon period generated in a park mode and a method for adjusting the same. The present application is based on Korean Patent Application No. 2001-75530, filed Nov. 30, 2001, which is incorporated herein by reference.
2. Description of the Prior Art
Bluetooth is a code name of a wireless data communication technology engaged in general fields of electrical communication, networking, computing and consumer goods. The Bluetooth technology can replace several cables required for an apparatus with a single wireless connection in a remote distance. For example, when the Bluetooth technology is realized in a mobile phone and a laptop computer, the apparatuses can be used without being connected with a cable. As the apparatuses that can be part of the Bluetooth system, there are a printer, a PDA (personal digital assistance), a desktop computer, a fax, a keyboard, and a joystick. As a matter of fact, all kinds of digital apparatuses can be part of the Bluetooth system.
Generally, the Bluetooth system has a maximum data transmission speed of 1 Mbps, and a maximum transmission distance of 10 m. 1 Mbps is a frequency in a region of the ISM (industrial scientific medical) frequency band of 2.4 GHz that a user can use without possessing a license, and is the transmission speed that can be easily realized with low costs. Moreover, the transmission distance 10 m is the result of a decision that 10 m is an adequate distance between an apparatus possessed by the user and a desktop computer in an office.
As the Bluetooth system has been designed to operate in a radio frequency environment having much noise, it employs a frequency hopping scheme of 1600 times per second for stable data transmission in a wireless frequency. Here, the frequency hopping method is one of a spread spectrum technology as well as a direct sequence method. The frequency hopping has one difference from the direct sequence method in the point that the frequency of a carrier signal is changed several times in every second.
The Bluetooth system supports not only one-to-one connections but also one-to-multiple connections. As shown in FIG. 1, a plurality of piconets can be formed and connected with one another. Each piconet is divided according to an order of the frequency hopping different from each other. In this case, the piconet is a Bluetooth network formed by a single master having one or more slaves connected thereto. The piconet can have a single master and a maximum of seven slaves. The master and the slaves communicate bilaterally by time division duplex (TDD) basically by using one hopping slot (625 μs= 1/1600) as a unit. A plurality of piconets systematically connected with one another is called a scatternet.
FIG. 2 is a view showing the communication by the TDD between the master and the slaves. Referring to FIG. 2, the length of each channel allocated to the time slot is 625 μs. The number of time slots is determined in accordance with a Bluetooth clock of the piconet master. The master and the slaves transmit the packet selectively in the time slots. In other words, the master transmits the packet only in the even-numbered time slots, and the slaves transmit the packet only in the odd-numbered time slots. Moreover, each of the packets transmitted to the master and the salves should be realized within five time slots. When multiple slots are transmitted, the hopping frequency of the time slot in which the transmission packet starts is used as the hopping frequency of all the packets. Here, the packet is a unit of data transmitted through the piconet channel.
To save power in the Bluetooth connection, the master can operate the slaves in a hold mode, a sniff mode, and a park mode. In the hold mode, a slave enters a sleep state, keeping an Active Member Address (AM_ADDR) and maintains connection with the master. In the sniff mode, the duty cycle of the slave's listen activity is reduced, while the slave keeps the AM_ADDR and the connection with the master. In the park mode, the slave gives up its active mode address AM_ADDR and enters a sleep state, while keeping the connection with the master. Accordingly, before entering into the park mode, the slave receives from the master a new address to be used in the park mode, such as a Parked Member Address (PM_ADDR) or an Access Request Address (AR_ADDR).
Here, the AM_ADDR is expressed as a member address, and distinguishes active members participating in the piconet. In other words, when two or more slaves are connected to a single master in the piconet, the master allocates a temporary 3-bit address to each slave, which will be used when the slaves wake up, to distinguish one slave from another. Therefore, all the packets transmitted between the master and the slaves carry AM_ADDR. In other words, the AM_ADDR of the slaves are used not only for the packet from the master to the slaves but also for the packet from the slaves to the master. When the slaves are not connected with the master, or when the slaves are in the park mode, the allocated AM_ADDR is given up. When the salves are re-connected with the master, a new AM_ADDR should be allocated. The reason for limiting the number of masters and slaves of the piconet to a single master and seven slaves is that the address AM_ADDR allocated by the master to the wakeup slaves is defined to be 3-bits according to the Bluetooth standard. In other words, among a maximum of eight addresses, the address ‘000’ is used for broadcasting from the master to the slaves, and only the seven addresses from ‘001’ to ‘111’ are used as the AM_ADDR.
The PM_ADDR distinguishes a parked slave from the other parked slaves. The parked slaves wake up at regular intervals to listen to the channel in order to re-synchronize and to check for broadcast messages. To support parked slaves, the master establishes a beacon channel when one or more slaves are parked.
FIG. 3 is a view showing a format of a general beacon channel. FIG. 4 is a view explaining an access window. Referring to FIGS. 3 and 4, the beacon channel consists of one beacon slot or a train of equidistant beacon slots which is transmitted periodically with a constant time interval. Referring to FIG. 3, the train of NB (NB≧1) beacon slots and the next train of beacon slots are defined with an interval of TB slots. The beacon slots in the train are separated by ΔB. The start of the first beacon slot of the train is called a beacon instant, and serves as the beacon timing reference. In other words, the train of beacon slots TB is the same as the beacon instant and the next beacon instant. Beacon parameters NB and TB are chosen such that there is sufficient time for the parked slaves to be synchronized during a predetermined time window in an error-prone environment. The beacon channel serves four purposes:
1) Transmission of master-to-slave packets which the parked slaves can use for re-synchronization;
2) carrying messages to the parked slaves to change the beacon parameters;
3) carrying general broadcast messages to the parked slaves;
4) unparking of one or more parked slaves.
The master of the piconet sets up the beacon access window by using various parameters to support the slaves in the park mode. During the window, the master gives the parked slaves, which want to be connected to the master, an opportunity for data transmission. This section appears periodically at the end of the beacon slot transmitted from the master to the slaves. The access window is defined where the parked slaves can send request signals to be unparked. To increase the reliability, the access window can be repeated Maccess times (Maccess≧1). The access window starts a fixed delay Daccess after the beacon instant until the start of the access window.
As described so far, in the current Bluetooth standard, it is not defined that the beacon parameter TB should be determined as a certain value. As the TB value is not determined, if the TB value is small, the beacon channel is frequently generated and the chance for the slaves in the park mode to be activated accordingly increases. However, the chance for currently activated slaves to transmit the data decreases as the chance for the parked slaves to be activated increases. Additionally, when the TB value is too great, the activated slaves can perform a normal transmission, but the parked slaves have to wait a long time to be connected with the master.